Variable cam timing (VCT) systems may be implemented in an engine to adjust a camshaft position that results in the camshaft timing being advanced or retarded. By controlling the cam timing with the VCT system, intake/exhaust valve timing may be varied based on engine operating conditions, such as engine load and engine speed. Such variable control facilitates increased engine performance, reduced emissions, and increased fuel efficiency compared to engines with fixed camshafts.
More particularly, electric VCT systems provide camshaft positioning functionality while the engine is cranking and firing events are not occurring. In other words, an electric VCT system can move a camshaft to a desired position for a first firing event during engine start-up. In one example, the electric VCT system adjusts the camshaft position based on closed-loop feedback that is calculated based on crankshaft position and camshaft profile ignition pickup (PIP) signals. These signals may be unavailable and/or unreliable until the engine speed meets a minimum threshold. Correspondingly, the closed-loop control of the camshaft is not available until the engine speed meets a minimum threshold. One way to avoid this issue is to move the camshaft to the desired position for the first firing event before engine shut-down. This way, the camshaft can be located at the desired position when the engine starts.
However, the inventors herein have recognized potential issues with such an approach. For example, the VCT system may not include locking pins to keep the camshaft in the desired position when there is no power to the VCT system. In such cases, closed loop feedback is not available when the engine speed drops below a certain threshold during engine shut-down and before the engine speed is above another threshold during engine start-up. Therefore, the position of the camshaft may be moved to a retarded position due to friction drag effects during the un-controlled phases, and thus would not be maintained at the desired location for the first engine firing event.
Thus, in one example, some of the above issues may be addressed by a method for controlling an electric VCT actuator. The method includes, during engine shutdown, adjusting a camshaft with an electric VCT actuator to a target starting location, applying a first current to the electric VCT actuator to maintain the target starting location, and during engine startup before camshaft feedback becomes available, applying a second current to the electric VCT actuator to maintain the target starting location. The first current may be a fixed current level irrespective of conditions, or it may be adjusted to various different levels depending on the conditions. Similarly, the second current may also be a fixed current level irrespective of conditions, or it may be adjusted to various different levels depending on the conditions, but it is nevertheless different than the first current for a given engine start procedure.
In one example, by providing the designated currents to the VCT actuator when the camshaft measurement is unavailable during engine shutdown and startup, the camshaft may be substantially prevented from drifting from the target starting position. In some embodiments, the current applied to the VCT actuator may differ between engine shutdown and engine startup because the valvetrain torque that is applied to the camshaft during engine shutdown and engine startup may differ. Further, when the camshaft position feedback becomes available, the accuracy of the camshaft position relative to the target starting position can be confirmed. In the event that the camshaft position is not at the target starting position, the electric VCT actuator can be controlled to quickly adjust the camshaft position to the target starting position since the camshaft is likely to be close to the target starting position. Thus, the time for the camshaft to achieve the target starting position after regaining feedback control may be substantially reduced, or in the ideal case reduced to zero, relative to an electric VCT system that does not provide different current levels for different conditions when the camshaft position feedback is unavailable.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.